1. Field of the Invention
The present invention relates to a rotation detecting device of a multi-rotation body, such as a steering wheel used in an automotive vehicle, which turns within a limited number of rotations, at least one rotation, and more particularly, to a rotation detecting device suitable for detecting the absolute position of the multi-rotation body.
2. Description of the Related Art
For the detection of rotation there has been proposed such a device designed to detect by an incremental encoder a precision steering-wheel angle of one rotation or less, and also to detect by an absolute encoder a coarse angle of one rotation or over of the steering wheel, so that the steering-wheel angle from a neutral position will be determined on the basis of a combination of these fine and coarse angles of rotation of the steering wheel.
The incremental encoder is comprised of a code plate which rotates integrally with a steering shaft, and a photo-interrupter oppositely disposed through the code plate. This encoder can detect only the turning angle of one rotation or less of the code plate; therefore the rotation of the code plate turning as one body with the steering shaft can be detected with high accuracy. On the other hand, the absolute encoder is comprised of a substrate having a resistance pattern thereon, a speed-reducing rotation body having a brush sliding on the resistance pattern, and a gearing for transmitting the rotation of the steering wheel to the speed-reducing rotation body. Since the rotation of the multi-rotation steering wheel is reduced through the gearing to one rotation or less, it is possible to detect the absolute position of the steering wheel.
In the above-described prior art, however, because the absolute encoder uses a gearing comprising a sun gear provided on a rotor member, a circular internal gear provided on a stator member, and a planetary gear engaged between the sun gear and the circular internal gear, there is such a drawback that a wide space is needed to mount the gearing which occupies the whole body of the rotation detecting device Furthermore, there also arises such a drawback that since the planetary gear is in constant mesh with both the sun gear and the circular internal gear, the speed-reducing rotation body supporting the planetary gear tends to rotate with a slight rotation of the steering wheel; particularly because the steering wheel is very often rotated (operated) in the vicinity of the neutral position, the resistance pattern is likely to get locally worn.
It is conceivable to use an optical detecting means to detect by a plurality of photo-interrupters the amount of rotation of the speed-reducing rotation body in place of the aforementioned resistance-type detecting means. In this case, however, one photo-interrupter is likely to be affected by an adjacent photo-interrupter, doing a mulfunction. Particularly there exists such a new drawback that when a small-sized rotation detecting device is employed to reduce a pitch of arrangement between the photo-interrupters, an adverse effect between adjacent photo-interrupters will become remarkable.